Organic light emitting full color display panel

ABSTRACT

A method for making full color display panel pixels is described. One aspect of the method relates to simultaneously depositing red, green, and blue dopants such that the blue dopant is dispersed in at least one non-blue subpixel. Another aspect of the method relates to using an integrated or removable mask to deposit components. In an angled evaporation method, the mask can correct for parallax.

TECHNICAL FIELD

This invention relates to full color organic light emitting displaydevices and methods of making them.

BACKGROUND

Organic light emitting devices (OLEDs) are useful for a variety ofapplications including displays, graphic signs, and lightingapplications. High resolution OLEDs that can provide a full range ofcolors are particularly desirable. Commercial development of full colorOLEDs requires patterning the light emitting area, i.e., the pixel, withthree primary colors (red, green, and blue) to provide a full colordisplay.

SUMMARY OF INVENTION

The present invention features a novel method of making an organic lightemitting color display panels using vacuum angle-evaporation techniques.

In one aspect of the invention, the inventors have found that,surprisingly, if a blue dopant is deposited with a red or green dopantduring the construction of a pixel, the red or green color willdominate. This is an important factor in producing a full-color lightemitting display. In another aspect, the inventors have found a way tocorrect for parallax during the making of a display panel. This is animportant factor in producing large high resolution display panels.

One aspect of the present invention provides a method of making organiclight emitting pixels having red, green, and blue subpixels on a displaypanel comprising:

for each pixel depositing a hole transporting layer and an electrontransporting layer; and depositing red, green, and blue dopantssimultaneously in a host layer such that the blue dopant is deposited onthe blue subpixel and at least one of the red and green subpixels. Thehost layer may be the hole transporting layer, the electron transportinglayer, or a layer between the hole transporting and electrontransporting layers.

Another aspect of the invention involves using a shadow mask during thedeposition process. The shadow mask may be integrated into the displaypanel, or may be removable and, optionally, reusable. The integratedmask may comprise photoresist, including dry film photoresist. Theremovable mask may be made from crystalline material (such as silicon),metal, or polymer.

In another aspect of the invention, the red and green dopant sources maybe located on opposite sides of the display at an angle of about 20° toabout 70°, typically 40°, from the pixel surfaces and the blue dopantand other material sources may be located in a plane that bisects thesubstrate and is normal to a straight line that connects the red andgreen dopant sources.

In another aspect of the invention, the deposition paths of the red andgreen dopants are isolated from each other and the other sources withshields that start at the red and green dopant sources and extend somedistance toward the pixel surface.

Another aspect of the invention provides a method of correcting forparallax in the making of an organic light emitting display panelcomprising using line-of-sight vapor deposition to create a series ofadjacent pixels, each pixel comprising sub-pixels, wherein one or moresources are positioned at an angle of about 20° to about 70°, typicallyabout 40°, from the pixel surfaces and wherein a shadow mask is used inthe deposition process, the mask having slots defined by ribs whereinthe pitch of the ribs is smaller than the pitch of the pixels.

Another aspect of the invention is an article comprising an organiclight emitting full color display panel wherein a blue dopant isdispersed over at least one non-blue sub-pixel.

Another aspect of the invention provides an organic light emitting colordisplay panel comprising: a plurality of full color pixels formed on asubstrate, each full color pixel comprising a red, a green, and a bluesubpixel, an integrated shadow mask, that corrects for parallax, forforming the color subpixels comprising a plurality of ribs erected onthe substrate, wherein the pitch of the ribs is smaller than the pitchof the pixels. The integrated mask may comprise photoresist material,including dry film photoresist.

Yet another aspect of the present invention provides a removable maskfor making an organic light emitting full color display panel by angledevaporation, the mask comprising a series of ribs that define slots inwhich individual pixels are built. The height of the ribs of the maskmay be approximately equal to the width of the pixels of the displaypanel. The mask may also have ribs with a pitch smaller than the pitchof the pixels on the substrate for which it will be used.

As used in this invention:

“display panel” means a two-dimensional array of individual pixels;

“parallax” or “parallax error” means the difference in shadow length atdifferent points on a substrate caused by the source being a finitedistance from the substrate;

“pitch” means the center to center distance between two adjacentstructures of the same type;

“pixel” means an area of an image display array that can be stimulatedto emit light independently of other areas; and

“sub-pixel” means an area of a pixel that can be addressed to emit lightof a particular color in a multi-color display.

An advantage of at least one embodiment of the present invention is thatred, green, and blue dopants and a host layer may be depositedsimultaneously in a single process step, which can make the constructionprocess faster.

Another advantage of at least one embodiment of the present invention isthat it provides a full-color display panel having high brightness, highcontrast, low manufacturing costs, and excellent visibility at allviewing angles.

Another advantage of at least one embodiment of the present invention isthat the removable shadow mask may be reused. Using a removable mask mayalso allow for a simpler process because it does not require applyingmaterial (e.g., photoresist) on the substrate to form a mask. Aremovable mask also results in a substantially planar substrate (afterdeposition and removal of the mask), which can be easier to use insubsequent processing steps.

Another advantage of at least one embodiment of the present invention isthat it provides a more efficient and reliable blue-emitting subpixelhaving a more desirable blue color.

Another advantage of at least one embodiment of the present invention isthat it allows for parallax correction, which is especially desirablefor making large display panels in a chamber of a limited size.

Advantages of at least one embodiment of the present invention thatincludes angle-evaporation of dopants (in comparison to having discreteemitting layers) for an OLED include that the operating voltages of eachof the color subpixels are nearly equal, the OLED has good quantumefficiency, improved reliability, excellent resolution betweensubpixels, and good color saturation.

Other features and advantages of the invention will be apparent from thefollowing drawings, detailed description, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a deposition system for constructing an organic lightemitting color display panel.

FIG. 2 depicts a cross section of a pixel and the angles at which red,blue, and green dopants are deposited.

FIG. 3 depicts parallax.

FIG. 4 depicts a mask that corrects for parallax.

DETAILED DESCRIPTION

The present invention provides methods of making organic light emittingcolor display panels.

As illustrated by system 10 in FIG. 1, an OLED can be made pursuant tothis invention by arranging sources 12 for the OLED structure, exceptfor the red and green dopant sources 14 and 16 (but including the bluedopant source 18) in a line near the bottom of a deposition chamber.Blue dopant source 18 is typically separated from the other OLEDsources, but is in the same line as the other sources. In general theOLED and blue dopant sources are located in a plane that bisects thedisplay panel substrate and is normal to an imaginary straight line thatconnects the red and green dopant sources. Typically an OLED isconstructed by depositing layers on a transparent conductor layer, butalternative constructions are also feasible, such as depositing atransparent electrode material on top of layers of organic materials.These different methods of construction are known in the art. Displaysubstrate 20 (e.g., a glass or polymer substrate) includes patternedelectrodes that define the emitting area for the subpixels. Theelectrodes may take the form of parallel lines for a passive-matrixdisplay, or individual subpixel areas connected to appropriate thin-filmtransistor driver circuits for an active-matrix display. The substrate,placed above (and typically in contact with) a shadow mask, can then bepositioned with the shadow mask ribs aligned parallel to and directlyover the line of sources. The red and green dopant sources are locatedaway from the line of sources such that the evaporant beams from the redand green sources impinge the substrate surface at an angle betweenabout 20° and about 70°, typically about 40°, from normal. Theappropriate angle will depend on the height and spacing of the ribs.

Applicants found that there is a further advantage in using shields 22to isolate a portion of the deposition path of each of the red and greendopants such that the dopant beams do not come into contact with thebeam of the host layer material and the blue dopant until they are inthe proximity of the pixel surface. By shielding the deposition paths,scattering of the dopant molecules is minimized so that each dopantcolor is concentrated in the desired subpixel. This minimization ofinter-beam scattering of the dopants provides good resolution of thesub-pixels at higher deposition rates.

Typically, the deposition chamber is evacuated to less than 1×10⁻⁵ Torr(1.3×10⁻³ Pa), or even less than 2×10⁻⁶ Torr (2.6×10⁻⁴ Pa). A lowpressure also helps to minimize scattering of the red and green dopantsinto the wrong subpixels, which would degrade the quality of thesubpixel colors.

FIG. 2 shows a construction 40 that can be made per the presentinvention. The figure depicts an OLED structure between ribs 41. Thelayers of the OLED are sequentially deposited at an angle near normal tothe substrate such that they cover nearly all of the area between theribs. Anode 42 of the OLED typically comprises a transparent conductor,such as indium tin oxide. Optional buffer layer 44 comprising e.g.,polypyrrole, poly(ethylenedioxythiophene) (PEDOT), or polyaniline, maybe deposited between anode 42 and hole injection layer 48. Optional holeinjection layer 46, e.g., copper Phthalocyanine (CuPc), may also bedeposited between anode 42 and hole injection layer 48. The OLED furthercomprises hole transport layer 48, e.g.,N,N′-Di(naphthalen-1-yl)-N,N′diphenylbenzidine(NPB), orN,N,N′,N′-tetrakis(4-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD);and electron transport layer 50, e.g.,(1,1′-Bisphenyl-4-Olato)bis(2-methyl-8-quinolinolato)Aluminum (BAlq), or4,4′-bis(2,2′-diphenylvinyl)biphenyl (DVPBi). Optional electroninjection layer 52, e.g., LiF, Li, or Li—Al, may be deposited beforecathode layer 54, e.g., Al, Ca, Ba, or Mg—Ag, which is subsequentlyapplied by vapor deposition. In this construction the electrontransporting layer functions as the host layer for the dopants. Red andgreen dopants are deposited in directions 56 and 57, respectively, indiscrete areas 50 a and 50 b of the host layer, while the blue dopant isdeposited in direction 58 throughout the host layer. In otherconstructions, other layers can serve as the dopant host, including thehole transporting layer or an additional host layer located between thehole transporting layer and the electron transporting layer. Thisaddition host layer may comprise, for example,4,4′-Bis(carbazol-9-yl)biphenyl (CBP)(see, for example, “Improved EnergyTransfer in Electrophosphorescent Devices,” D. F. O'Brien, M. A. Baldo,M. E. Thompson, and S. R. Forrest, Applied Physics Letters, Jan. 18,1999, Volume 74, Issue 3 pp. 442-44) or other hole transporting orelectron transporting materials.

One aspect of the present invention comprises simultaneously depositingred, blue, and green dopants when the host layer is deposited. Thedopants are deposited directionally as indicated in FIG. 2 so that thered and green dopants are deposited over a single subpixel and the bluedopant may be deposited over all three subpixels. Although it wasunexpected, the applicants found that the color of the red and greendopants dominated the color of the simultaneously deposited blue dopantthereby allowing the blue dopant to be deposited over one or both of thered and green subpixels without interfering with the quality and clarityof the OLED.

Red dopants suitable for use in the present invention include, e.g.,platinum octaethylporphyrin (PtOEP), or4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB).

Blue dopants suitable for use in the present invention include, e.g.,perylene.

Green dopants suitable for use in the present invention include, e.g.,10-(2-bensothiazolyl)-2,3,6,7,tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7,8-i j) quinolizin-11-one (C545T), N,N-dimethyl quinacridone (DMQA),or tris(2-phenylpyridine)iridium (Ir(ppy)₃).

The dopant concentrations in the host layer are typically on the orderof about 1 molar percent. Optionally, a second electron transportinglayer (not shown), e.g., tris(8-hydroxy-quinolinato) aluminum (Alq3),may be deposited over a doped electron transport layer to improveelectron injection.

As previously mentioned, an aspect of the present invention involvesdepositing dopants using an angle evaporation technique that employs ashadow mask. A shadow mask may be integrated into the display panel suchthat the ribs are constructed on, or as part of, the substrate to definethe outer edges of a pixel. Alternatively, the shadow mask may beremovable and reusable. With a removable mask, ribs define a slotthrough which materials may be vapor deposited onto the substrate.

The ribs of a mask can partially block the deposition of a dopant sothat only a portion of the pixel receives the dopant, thereby allowingthe formation of red, green, and blue sub-pixels. (This is generallyillustrated in FIG. 2.) For example, a source for a red dopant may belocated to the left of a pixel at an angle of about 20° to about 70°from normal to the substrate such that, due to the location and heightof a rib on the left side of the pixel, the red dopant is only depositedon the right third portion of the pixel (i.e., the right subpixel).Similar positioning may be done on the right side of the pixel for thegreen dopant source. Because of the applicants' discovery that emissionfrom the red and green dopants will dominate the emission from asimultaneously deposited blue dopant, the blue dopant may be depositedat an angle normal to the surface of the pixel, in relation to thelocation of the other dopant sources, so that it covers the entirepixel.

The height of the ribs of the mask will determine the length of theshadow cast. The desired length of the shadow can depend on factors suchas the angle at which the deposition source is placed, the distance ofthe deposition source from the pixel, and the area desired to be coveredby the evaporant. In most cases, the preferred height of the ribs iscomparable to the width of the pixel.

As previously stated, the masks of the present invention may bepermanently attached to the display devices or may be removable. Apermanently attached, i.e., integrated, photoresist shadow mask may bemade by conventional photolithography, see, e.g., U.S. Pat. Nos.5,294,869, and 5,294,870, incorporated by reference. The photoresist maybe a dry film photoresist, which is a film that can be applied with aroller. Dry film photoresist is available as item # MP130 fromMacDermid, Waterbury, Conn.

A removable mask comprises a series of slots that are defined by ribs.Materials comprising pixels are deposited through the slots. Materialssuitable for makine a removable mask include crystalline materials suchas silicon, metal materials such as copper and steel, and polymericmaterials. A removable mask is typically made by removing material wherethe slots will be located, while leaving intact the material that formsthe ribs. A removable mask may be made by a variety of techniques,including conventional machining: micromachining; diamond machining;laser ablation; or chemical, plasma, or ion beam etching (typicallypatterned by photolithography). Electric discharge machining (EDM), alsocalled electrodischarge or spark-erosion machining, is a well-knowntechnique that can be used for making a mask. EDM works by erodingmaterial in the path of electrical discharges that form an arc betweenan electrode tool (in this case a wire) and the work piece.

Wafers of crystalline materials (e.g., silicon, germanium, or galliumarsenide) are particularly well suited for making removable, reusablemasks for angle evaporation. Silicon wafers of appropriate thickness(e.g., 100-200 μm) and polished on both sides, are widely available. Theribs required for use as an angle evaporation mask can be fabricatedwith well known processes including standard photolithography andetching. The pattern may be etched through the wafer by any appropriatetechnique including anisotropic wet chemical etching (see Marc Madou,Fundamentals of Microfabrication, CRC Press, 1997, p. 168-176), oranisotropic ion etching (see U.S. Pat. No. 5,501,893).

In making display panels, the deposition sources for the OLED materials(including the blue dopant) are approximately point sources or linesources. The red and green dopant sources are approximately pointsources located at an angle at the sides of the substrate. FIG. 3illustrates a common problem in angle deposition known as parallax.Because dopant source 62 is a finite distance from the substrate, thehorizontal lengths of the shadows cast by each set of ribs 64 will varyacross the width of the substrate due to parallax effects. The ribs of astandard shadow mask have the same pixel pitch 66, i.e., distance fromcenter to center of a pair of ribs, as rib pitch 68, i.e., distance fromcenter to center of a pair of pixels. Parallax error can cause angleddeposition sources for individual pixels and subpixel electrodes 70 on asingle display panel to be misaligned with the subpixel electrodes onwhich they are to be deposited.

As illustrated by FIG. 4, an aspect of the present invention correctsparallax error by providing precise alignment of the angled dopantsources being deposited through ribs 64 with subpixel electrodes 70patterned on the substrate by making pixel pitch 66 slightly larger thanrib pitch 68 on the mask according to the following formula:

p′=p(1+h/d)

where p′ is the pitch of the pixels (corresponding to the pitch of theelectrodes),

p is the pitch of the ribs of the shadow mask,

d is the height of the substrate above the source, and

h is the height (i.e., thickness) of the shadow mask.

In using the masks of the present invention, the source of the dopantsis preferably separated from the pixels by a distance that is at least 5times the width of the display.

The invention may be illustrated by way of the following examples.

EXAMPLES

This invention may be illustrated by way of the following example.

In this example, a passive-matrix OLED display was fabricated by angleevaporation of OLED materials through a removable metal shadow mask. TheOLED was built on a glass substrate with a 140 nm thick coating ofindium tin oxide (ITO) transparent conductor, provided by Thin FilmDevices (Anaheim, Calif.). The ITO was etched into column electrodesusing conventional photoresist patterning and etching in warm (60° C.),concentrated HCl. The pixels were arranged in a 0.075″ (1.905 mm) by0.075″ (1.905 mm) grid. Three ITO columns were located under each pixel,corresponding to red, blue and green subpixels. Each column wasnominally 0.011″ (280 μm) wide, with a gap of 0.0055″ (140 μm) betweenthe subpixel columns.

A removable and reusable metal shadow mask was fabricated from a steelplate (0.047″ (1.194 mm) thick) using wire electric discharge machining(Wire EDM). Slots machined into the steel plate measured 0.065″ (1.651mm) wide and 0.0746″ (1.895 mm) center-to-center (i.e., pitch), leavingribs having a width of 0.0096″ (0.244 mm) (with a pitch of 0.0746″(1.895 mm). and a height of 0.047″ (1.194 mm)). The small difference inpitch between the metal mask (0.0746″ (1.895 mm)) and the ITO columns(0.075″ (1.895 mm)) were appropriate to compensate for parallax in anevaporator system in which the substrate was positioned about 9″ (229mm) above the sources.

The substrate with etched ITO lines was coated with a spun-on conductivepolymer buffer layer of polyethylenedioxythiophene such as Baytron Pavailable from Bayer (Pittsburgh, Pa.) and dried on a hot plate (100°C.) in a nitrogen atmosphere. The substrate was then placed on the metalmask and the ITO columns were aligned with the slots in the mask. Themask and substrate were clamped together and positioned in the vacuumevaporator system, which was evacuated to approximately 10⁻⁶ torr(1.3×10⁻⁴ Pa). A hole transporting layer (HTL) was first applied (NPB)with an approximate thickness of 30 nm. Then an electron transportinglayer (ETL) was applied (BAlq) which also acted as a host for thedopants. Approximately 20 nm of the ETL nearest the HTL was doped,followed by approximately 20 nm of undoped ETL. The dopants used wereperylene (blue), C545T (green) and PtOEP (red). The HTL, ETL, and bluedopant sources were arranged at the bottom of the evaporator chamber, ina line directly beneath and parallel to the ITO columns. The red andgreen dopant sources were placed some distance from that line, so thatthe evaporant beams from those dopant sources impinged the substrate atan angle of about 40° from normal. Ribs of the mask cast a shadow suchthat the green and red dopants impinged only on the appropriate ITOsubpixel columns, and not on the subpixel columns for the other colors.The blue dopant was deposited on all three subpixels, but in the greenand red subpixels, the green and red dopants effectively dominated theemission spectrum so that any blue emission from those subpixels wasinconsequential.

After deposition of these organic materials, the evaporator chamber wasvented, and the shadow mask was removed from the substrate and replacedwith another shadow mask with slots that ran orthogonal to the ITOcolunns. This second shadow mask was used to pattern rows of cathodeelectrodes in a second vacuum evaporation process. The cathode wasformed by deposition of 0.5 nm of LiF followed by 200 nm of Al.Alternatively, the cathode could have been formed with 20 nm of Cafollowed by 200 nm of Al.

After deposition of the cathode, the substrate was removed from thedeposition chamber and the cathode shadow mask was removed. The displaywas essentially complete, and ready for encapsulation. Thispassive-matrix display was operated by sequentially applying a voltageto each of the cathode rows and simultaneously addressing each of thesubpixels with the appropriate current to provide the light emissiondesired for each row of the display as it was addressed.

Various modifications and alterations to this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention. It should be understood that thisinvention is not intended to be unduly limited by the illustrativeembodiments and examples set forth herein and that such examples andembodiments are presented by way of example only with the scope of theinvention intended to be limited only by the claims set forth herein asfollows.

What is claimed is:
 1. An article comprising an organic light emittingfull color display panel wherein red, green and blue dopants aresimultaneously deposited in a host layer such that the blue dopant isdeposited on a blue sub-pixel and at least one of a red and greensub-pixel, and wherein the red and green dopants emit light throughelectroluminenscence.
 2. The article of claim 1 wherein the blue dopantis dispersed in an electron transporting layer.
 3. The article of claim1 wherein the blue dopant is dispersed in a hole transporting layer. 4.The article of claim 1 wherein the blue dopant is dispersed in a hostlayer between an electron transporting layer and a hole transportinglayer.
 5. The article of claim 1 wherein a mask is integrated with thedisplay panel.
 6. The article of claim 5 wherein the mask comprisesphotoresist.
 7. The article of claim 6 wherein the photoresist is a dryfilm photoresist.